1. Field of Invention
This invention relates to a laminated polyester resin container and the method of molding such a container. More particularly, this invention relates to a laminated, biaxially oriented container of a thermoplastic polyester, including polyethylene terephthalate as the major component, which is laminated with a thin, clear film layer having high gas barrier properties; and to the method of molding such a container.
2. Description of Related Art
The polyethylene terephthalate resin (hereinafter referred to as PET resin) is a representative thermoplastic polyester resin known to have stable physical properties, antipollution property, an outstanding degree of transparency, and a high mechanical strength. Because of these favorable properties, the PET is used in large amounts in various fields of application as the biaxially oriented, blow-molded bottles and hollow containers in other shapes.
Especially because these PET containers contain no additive such as plasticizer or stabilizer and are hygienic and harmless to the human body, they attract attention, and are in wide use, as quite useful containers for medical use and for foods and drinks.
Although PET resin containers have so many quite excellent properties, even PET resin containers fall short of the gas barrier property against oxygen in the air, especially in cases where the foods, with which bottles are filled, require a high air-blocking property. Because of such low gas barrier properties, the food may lose its taste or food spoilage may result. Thus, an unsatisfactory point still remains.
One of the methods of solving these various problems is to mold a multilayer preform by laminating a layer of a different type of resin having a high air-blocking property on the outer surface of the polyethylene terephthalate resin layer and then blow molding this preform in the biaxial orientation to obtain a multilayer container.
However, the hollow containers thus obtained were not acceptable as commercial products because the crystallizing and/or clouding phenomenon occurs between the inner and outer layers when the preform is obtained by laminating different resins, one by one, by the conventional injection molding method. As a result, the blow moldability of the preform gets worse and its adhesive property decreases at the interface between resin layers.
Official gazettes of Japanese patent applications laid open No. 1982-128516 and No. 1990-258310 proposed the well-known improvements in the preparation of multilayer preforms that are obtained by almost simultaneous lamination of a PET resin with another resin. These methods involved injecting the polyethylene terephthalate resin into the mold and immediately thereafter co-injecting into the same mold one of various nylons (e.g., an MXD-6 nylon resin), which are polyamide resins containing meta-xylyl radicals and having high gas barrier properties, to mold the preform of a 3-layer structure comprising the inner and outer layers of the polyethylene terephthalate resin and the middle layer of a nylon resin. The 3-layer preform is then blow molded into a multilayer container.
Although MXD-6 nylon and ethylene vinyl alcohol copolymer resins have high gas barrier properties, they have also unfavorable mechanical properties and low degrees of transparency. It is desirable to make the gas barrier layer as thin as possible and to secure the transparency of laminates as much as possible. When the multilayer preform were blow molded under these limited conditions, the hollow container thus obtained would often have insufficient gas barrier properties, as evidenced by the breaking in the resin layers that have high gas barrier properties but poor mechanical strength.
Meanwhile, a thin gas barrier resin layer in the above-described 3-layer structure was found difficult to obtain. It was also pointed out that the containers obtained from the above-described 3-layer preforms were disadvantageous because, when gases pass through a thermoplastic polyester resin layer and is blocked by the gas barrier layer, the gases accumulate between these layers over time, thus causing the cleavage of layers. Official gazettes of Japanese patent application laid open No. 1985-240409 and patent publication No. 1993-79494 proposed to inject the PET resin, an MX nylon resin, and the PET resin in this order into the same mold to mold a 5-layer preform comprising the inner and outer layers of polyethylene terephthalate as well as the center core of PET, plus two thin intermediate layers of the MXD nylon resin in an alternately laminated structure.
Following is a quotation from these inventions: “This injection molding can be conducted by using an injection-molding machine provided with two injection cylinders, i.e., one for injecting a thermoplastic polyester resin (resin A) and the other for injecting a polyamide resin (resin B) containing meta-xylyl radicals. Molten resins A and B are injected, one by one, into the same mold in the order of resin A, resin B, and resin A under the conditions that satisfy the equations (1) to (4) described below. The trunk portion of the preform (corresponding to the bottle trunk after blow molding) has a 5-layer structure which comprises the inner and outer layers of resin A as well as the center core of resin A, plus two intermediate layers of resin B that sandwich the center core, with at least the open mouth portion having a single-layer structure. A multilayer container can be obtained by blow molding such a preform in the biaxial orientation.V1≧V2  (1)8 cc/sec≦V2≦35 cc/sec  (2)0.7≦A1/A2≦1.6  (3)B1/(A1+A2+B1)≦0.25  (4)“Wherein V1 is the injection speed for the initially injected resin A; V2 is the injection speed for the lastly injected resin A; A1 is the injection shot volume for the initially injected resin A; A2 is the injection shot volume for the lastly injected resin A; and B1 is the injection shot volume for the resin B. Cleavage can be prevented by providing two gas barrier resin layers and dispersing gasses that accumulate between layers.”
The above-described 3-layer and 5-layer preforms are preheated to a blow moldable temperature, and then are expanded in a blow mold and blown in the biaxial orientation to obtain multilayer containers. However, even the containers of the 5-layer structure cannot be prevented from the breaking of the gas barrier layer observed in the above 3-layer structure. Under these circumstances, the above-described multilayer containers are not totally reliable and are not yet in the actual use.
This invention has been made to solve these problems. An object of this invention is to provide a laminated, biaxially oriented container of the multilayer structure comprising thermoplastic polyester resin layers and a thin, clear intermediate layer or layers having gas barrier properties, wherein insufficient reliability of the thin, clear gas barrier layer or layers are complemented by the impartment of gas barrier properties to the container wall so that the gas barrier properties of the container can be secured. Another object of this invention is to provide a method of molding such a laminated polyester resin container.